The present invention relates to a heat developable light-sensitive material, a heat development image forming process, a thermal image recording material, a thermally decoloring image recording process and a process for decoloring cyanine dye.
A heat developable light-sensitive material (or a photothermographic material) has already been proposed, and is described in U.S. Pat. Nos. 3,152,904, 3,457,075, and B. Shely xe2x80x9cThermally Processed Silver Systemsxe2x80x9d (Imaging Processes and Materials, Neblette eighth edition, edited by Sturge, V. Walworth and A. Shepp, page 2, 1996).
The heat developable light-sensitive material generally has a light-sensitive layer, which contains a catalytically active amount of a photo catalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver salt) and a color toning agent dispersed in a binder matrix. The color toning agent has a function of controlling color tone of silver. A heat development image forming process comprises steps of imagewise exposing to light the heat developable light-sensitive material, and then heating the light-sensitive material at an elevated temperature (not lower than 80xc2x0 C.) to cause an oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the reducing agent. Thus a black silver image is formed. The oxidation-reduction reaction is accelerated by a catalytic function of a silver halide latent image formed at the exposing step. Accordingly, the black silver image is formed within the exposed area.
The heat development does not require processing solutions of a wet development. The heat development can easily and rapidly be conducted, compared with the wet development. However, the wet development is still a major photographic technology, while the heat development is minor. The heat development has unsolved problems, while the wet development does not have the problems.
A photographic material usually contains a dye, such as a filter dye, an antihalation dye or an antiirradiation dye. The dye functions at the exposing step. If the dye remains in the photographic material after image formation, a formed image would be colored with the dye. Therefore, the dye should be removed from a photographic material at a developing step. At the wet development, the dye can easily be removed from a photographic material by using processing solutions. On the other hand, it is very difficult (substantially impossible) to remove the dye from a photographic material at the heat development.
A simple, easy and rapid development has been desired in the field of recent photography, especially in the field of recent clinical or printing photography. The improvement of the conventional wet development, however, has nearly reached its limits. Therefore, much attention has been paid again to a heat development image forming process in the field of clinical or printing photography.
Since it is very difficult to remove a dye at the heat development, it has been proposed to decolor the dye at the heat development. For example, U.S. Pat. No. 5,135,842 discloses a method of decoloring a polymethine dye of a specific structure by heating a photographic material. U.S. Pat. Nos. 5,314,795, 5,324,627 and 5,384,237 disclose a method of decoloring a polymethine dye by heating a photographic material in the presence of a carbanion forming agent (nucleophilic agent).
The known process of decoloring a dye by heat has some problems. For example, some dyes are not sufficiently decolored at heat development. Other dyes are decolored while storing a heat developable light-sensitive material because the dyes are not stable. Further, some known dyes are decolored to form decomposition products that have light absorption. Therefore, a formed image (particularly highlighted area) is colored with the decomposition products. Furthermore, some decolored dyes are colored again after the heat development (particularly in the presence of an acid). Moreover, a process of decoloring a dye with another compound such as a nucleophilic agent is influenced with a (stoichiometrical or dimensional) relation between the dye and the agent. Accordingly, the decoloring reaction between the dye and the agent is relatively slow.
An object of the present invention is to provide a heat developable light-sensitive material containing a dye that is free from the above-mentioned problems.
Another object of the invention is to provide a heat development image forming method that can form a clear image in which the dye is completely decolored.
A further object of the invention is to provide a new thermal image recording material.
A furthermore object of the invention is to provide a thermally decoloring image forming process that forms a decolored image in a simple manner.
A still further object of the invention is to provide a process of decoloring a dye that is stable at room temperature by a substantially irreversible quick reaction.
The present invention provides a heat developable light-sensitive material comprising a support, a light-sensitive layer and a non-light-sensitive layer, said light-sensitive layer containing silver halide and a reducing agent, and said non-light-sensitive layer containing a cyanine dye represented by the formula (I) or a salt thereof and a base precursor: 
in which R1 is hydrogen, an aliphatic group, an aromatic group, xe2x80x94NR21R24, xe2x80x94OR21 or xe2x80x94SR21, each of R21 and R24 independently is hydrogen, an aliphatic group or an aromatic group, or R21 and R24 are combined to form a nitrogen-containing heterocyclic ring; R2 is hydrogen, an aliphatic group or an aromatic group; R3 is an aliphatic group; L1 is a methine chain consisting of an odd number of methines; and each of Z1 and Z2 independently is an atomic group forming a five-membered or six-membered nitrogen-containing heterocyclic ring, which may be condensed with an aromatic ring.
The invention also provides a heat development image forming process comprising steps of:
imagewise exposing to light a heat developable light-sensitive material comprising a support, a light-sensitive layer and a non-light-sensitive layer, said light-sensitive layer containing silver halide and a reducing agent, and said non-light-sensitive layer containing a cyanine dye represented by the formula (I) or a salt thereof and a base precursor: and then
heating the heat developable light-sensitive material at 80 to 200xc2x0 C. to form a base from the base precursor whereby the cyanine dye is decolored and to develop the silver halide.
The invention further provides a thermal image recording material comprising a support and an image recording layer, said image recording layer containing a cyanine dye represented by the formula (I) or a salt thereof and a base precursor.
The invention furthermore provides a thermally decoloring image recording process comprising imagewise heating a thermal image recording material at 80 to 200xc2x0 C., said image recording material comprising a support and an image recording layer, said image recording layer containing a cyanine dye represented by the formula (I) or a salt thereof and a base precursor to form a base from the base precursor whereby the cyanine dye is decolored.
The invention still further provides a process for decoloring a cyanine dye comprising heating a cyanine dye represented by the formula (II) or a salt thereof at 80 to 200xc2x0 C. in the presence of a base: 
in which X21 is xe2x80x94NR24xe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94; each of R21 and R24 independently is hydrogen, an aliphatic group or an aromatic group, or R21 and R24 are combined to form a nitrogen-containing heterocyclic ring; R22 is hydrogen, an aliphatic group or an aromatic group; R23 is an aliphatic group; L21 is a methine chain consisting of an odd number of methines; and each of Z21 and Z22 independently is an atomic group forming a five-membered or six-membered nitrogen-containing heterocyclic ring, which may be condensed with an aromatic ring.
The present inventors have found that the cyanine dye represented by the formula (I) is advantageously added to a non-light-sensitive layer of a heat developable light-sensitive material. The cyanine dye represented by the formula (I) is quickly decolored by a substantially irreversible reaction at heat development in an image forming method. According to study of the present inventors, a substantially colorless compound is formed from the cyanine dye represented by the formula (I) by an intramolecular ring forming reaction when the dye is heated in the presence of a base (under a basic condition). The reaction rapidly proceeds without influence caused by another agent because the decoloring reaction is an intramolecular reaction. Further, the decoloring reaction is a ring forming reaction that forms a five-membered or seven-membered ring condensed with the basic nucleus (onium form) of the cyanine dye. The formed compound is substantially colorless and relatively stable. Accordingly, the decoloring reaction is substantially irreversible. For the reasons mentioned above, the heat developable light-sensitive material of the present invention can form a clear image in which the dye is completely decolored.
Further, a thermal image recording material can be prepared by using the cyanine dye represented by the formula (I). A thermally decolored image can be easily formed by a simple step of imagewise heating the thermal image recording material.
Furthermore, the cyanine dye represented by the formula (II) is a stable compound at room temperature. According to the process of decoloring a dye, the stable dye can be decolored by a substantially irreversible quick reaction.
The present invention uses a cyanine dye represented by the formula (I) or a salt thereof. 
In the formula (I), R1 is hydrogen, an aliphatic group, an aromatic group, xe2x80x94NR21R24, xe2x80x94OR21 or xe2x80x94SR21. Each of R21 and R24 independently is hydrogen, an aliphatic group or an aromatic group, or R21 and R24 are combined to form a nitrogen-containing heterocyclic ring. R1 preferably is xe2x80x94NR21R24, xe2x80x94OR21 or xe2x80x94SR21, as is defined in the formula (II). The details of xe2x80x94NR21R24, xe2x80x94OR21 and xe2x80x94SR21 are described about the formula (II).
In the present specification, the aliphatic group means an alkyl group, a substituted alkyl group, an alkenyl group, a substituted alkenyl group, an alkynyl group, a substituted alkynyl group, an aralkyl group and a substituted aralkyl group. The alkyl group, the substituted alkyl group, the alkenyl group, the substituted alkenyl group, the aralkyl group and the substituted aralkyl group are preferred, and the alkyl group, the substituted alkyl group, the aralkyl group and the substituted aralkyl group are more preferred. The aliphatic group preferably has a chain structure rather than a cyclic structure. The aliphatic group of the chain structure may be branched.
The alkyl group preferably has 1 to 30 carbon atoms, more preferably has 1 to 20 carbon atoms, further preferably has 1 to 15 carbon atoms, and most preferably has 1 to 12 carbon atoms. The alkyl moiety of the substituted alkyl group is the same as the above-described alkyl group.
The alkenyl group and the alkynyl group preferably has 2 to 30 carbon atoms, more preferably has 2 to 20 carbon atoms, further preferably has 2 to 15 carbon atoms, and most preferably has 2 to 12 carbon atoms. The alkenyl moiety of the substituted alkenyl group and the alkynyl moiety of the substituted alkynyl group are the same as the above-described alkenyl group and alkynyl group respectively.
The aralkyl group preferably has 7 to 35 carbon atoms, more preferably has 7 to 25 carbon atoms, further preferably has 7 to 20 carbon atoms, and most preferably has 7 to 15 carbon atoms. The aralkyl moiety of the substituted aralkyl group is the same as the above-described aralkyl group.
Examples of the substituent groups of the aliphatic groups (the substituted alkyl group, the substituted alkenyl group, the substituted alkynyl group and the substituted aralkyl group) include a halogen atom (fluorine, chlorine, bromine), hydroxyl, nitro, carboxyl, sulfo, an acyl group, an alkoxy group, an alkoxycarbonyl group, an alkylthio group, an alkylthiocarbonyl group, an aryloxy group, an aryloxycarbonyl group and a carbamoyl group. Carboxyl and sulfo can be in the form of a salt. The cation forming a salt with carboxyl or sulfo preferably is an alkali metal ion (e.g., sodium ion, potassium ion).
In the present specification, the aromatic group means an aryl group and a substituted aryl group.
The aryl group preferably has 6 to 30 carbon atom, more preferably has 6 to 20 carbon atoms, further preferably has 6 to 15 carbon atoms, and most preferably has 6 to 12 carbon atoms. The aryl moiety of the substituted aryl group is the same as the above-described aryl group.
Examples of the substituent groups of the aromatic group (the substituted aryl group) include a halogen atom (fluorine, chlorine, bromine), hydroxyl, nitro, carboxyl, sulfo, an alkyl group, an acyl group, an alkoxy group, an alkoxycarbonyl group, an alkylthio group, an alkylthiocarbonyl group, an aryloxy group, an aryloxycarbonyl group and a carbamoyl group. Carboxyl and sulfo can be in the form of a salt. The cation forming a salt with carboxyl or sulfo preferably is an alkali metal ion (e.g., sodium ion, potassium ion).
In the formula (I), R2 is hydrogen, an aliphatic group or an aromatic group. The aliphatic group and the aromatic group are defined above. R2 preferably is hydrogen or an aliphatic group, more preferably is hydrogen or an alkyl group, further preferably is hydrogen or an alkyl group having 1 to 15 carbon atoms, and most preferably is hydrogen.
In the formula (I), R3 is an aliphatic group. The aliphatic group is defined above. R3 preferably is a substituted alkyl group. In view of synthesis of the compound, R3 preferably is a substituted alkyl group having the same meanings as xe2x80x94CHR2xe2x80x94COxe2x80x94R1.
In the formula (I), L1 is a methine chain consisting of an odd number of methines. The number of the methines preferably is 3, 5, 7 or 9, more preferably is 3, 5 or 7, further preferably is 5 or 7, and most preferably is 5.
The methine may have a substituent group. Examples of the substituent groups include a halogen atom, an aliphatic group, an aromatic group, xe2x80x94NR5R6, xe2x80x94OR5 and xe2x80x94SR5. Each of R5 and R6 independently is hydrogen, an aliphatic group or an aromatic group. The aliphatic group and the aromatic group are defined above. The substituent groups of the methine can be combined to form an unsaturated aliphatic ring or an unsaturated heterocyclic ring. The unsaturated aliphatic ring is preferred to the unsaturated heterocyclic ring. The formed ring preferably is a five-membered or six-membered ring. Cycloheptene ring is particulary preferred. The methane chain preferably is not substituted, or forms cyclohexane ring by combining substituent groups.
In the formula (I), each of Z1 and Z2 independently is an atomic group forming a five-membered or six-membered nitrogen-containing heterocyclic ring. Examples of the nitrogen-containing heterocyclic rings include oxazole ring, thiazole ring, selenazole ring, pyrroline ring, imidazole ring and pyridine ring. A six-membered ring is preferred to a five-membered ring. The nitrogen-containing heterocyclic ring may be condensed with an aromatic ring (benzene ring, naphthalene ring). The nitrogen-containing heterocyclic ring and the condensed ring may have a substituent group. Examples of the substituent groups include a halogen atom (fluorine, chlorine, bromine), hydroxyl, nitro, carboxyl, sulfo and an alkyl group. Carboxyl and sulfo can be in the form of a salt. The cation forming a salt with carboxyl or sulfo preferably is an alkali metal ion (e.g., sodium ion, potassium ion).
The cyanine dye represented by the formula (I) is preferably used in the form of a salt, which consists of the dye and an anion. In the case that the cyanine dye represented by the formula (I) has an anionic group such as carboxyl and sulfo, the dye can form an intramolecular salt. In the other cases, the cyanine dye preferably forms a salt with an anion other than its molecule. The anion preferably is monovalent or divalent, and more preferably is monovalent. Examples of the anions include halide ion (Cl, Br, I), p-toluenesulfonate ion, ethylsulfate ion, 1,5-disulfonaphthalene dianion, PF6, BF4 and ClO4.
A preferred cyanine dye is represented by the formula (Ia). 
In the formula (Ia), R11 is hydrogen, an aliphatic group, an aromatic group, xe2x80x94NR31R34, xe2x80x94OR31 or xe2x80x94SR31. Each of R31 and R34 independently is hydrogen, an aliphatic group or an aromatic group, or R31 and R34 are combined to form a nitrogen-containing heterocyclic ring. R11 preferably is xe2x80x94NR31R34, xe2x80x94OR31 or xe2x80x94SR31, as is defined in the formula (IIa). The details of xe2x80x94NR31R34, xe2x80x94OR31 and xe2x80x94SR31 are described about the formula (IIa).
In the formula (Ia), R12 is hydrogen, an aliphatic group or an aromatic group. R12 preferably is hydrogen or an aliphatic group, more preferably is hydrogen or an alkyl group, further preferably is hydrogen or an alkyl group having 1 to 15 carbon atoms, and most preferably is hydrogen.
In the formula (Ia), R13 is an aliphatic group. R13 preferably is a substituted alkyl group. In view of synthesis of the compound, R13 preferably is a substituted alkyl group having the same meanings as xe2x80x94CHR12xe2x80x94COxe2x80x94R11.
In the formula (Ia), L11 is a methine chain consisting of an odd number of methines. The number of the methines preferably is 3, 5, 7 or 9, more preferably is 3, 5 or 7, further preferably is 5 or 7, and most preferably is 5.
The methine may have a substituent group. Examples of the substituent groups include a halogen atom, an aliphatic group, an aromatic group, xe2x80x94NR15R16, xe2x80x94OR15 and xe2x80x94SR15. Each of R15 and R16 independently is hydrogen, an aliphatic group or an aromatic group. The aliphatic group and the aromatic group are defined above. The substituent groups of the methine can be combined to form an unsaturated aliphatic ring or an unsaturated heterocyclic ring. The unsaturated aliphatic ring is preferred to the unsaturated heterocyclic ring. The formed ring preferably is a five-membered or six-membered ring. Cycloheptene ring and cyclohexene ring are particularly preferred. The methine chain preferably is not substituted, or forms cycloheptene ring or cyclohexene ring by combining substituent groups.
In the formula (Ia), each of Y11 and Y12 independently is xe2x80x94CR14R15xe2x80x94, xe2x80x94NR14xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94Sexe2x80x94. Each of R14 and R15 independently is hydrogen or an aliphatic group or R14 and R15 are combined to form an aliphatic ring. The aliphatic group preferably is an alkyl group or a substituted alkyl group. The aliphatic ring preferably is a saturated aliphatic ring, more preferably is five-membered ring (cyclopentane ring), six-membered ring (cyclohexane ring) or seven-membered ring (cycloheptane ring), and most preferably is cyclohexane ring.
In the formula (Ia), the benzene rings of Z11 and Z12 may be condensed with another benzene ring. The benzene rings of Z11, Z12 and the condensed ring may have a substituent group. Examples of the substituent groups include a halogen atom (fluorine, chlorine, bromine), hydroxyl, nitro, carboxyl, sulfo and an alkyl group. Carboxyl and sulfo can be in the form of a salt. The cation forming a salt with carboxyl or sulfo preferably is an alkali metal ion (e.g., sodium ion, potassium ion).
The cyanine dye represented by the formula (Ia) is preferably used in the form of a salt, which consists of the dye and an anion. The salt is described about the formula (I).
A more preferred cyanine dye is represented by the formula (Ib). 
In the formula (Ib), the two groups of R41 are identical. R41 is hydrogen, an aliphatic group, an aromatic group, xe2x80x94NR51R52, xe2x80x94OR51 or xe2x80x94SR51. Each of R51 and R52 independently is hydrogen, an aliphatic group or an aromatic group, or R51 and R52 are combined to form a nitrogen-containing heterocyclic ring. R41 preferably is xe2x80x94NR51R52, xe2x80x94OR51 or xe2x80x94SR51, as is defined in the formula (IIa). The details of xe2x80x94NR51R52, xe2x80x94OR51 and xe2x80x94SR51 are described about the formula (IIb).
The cyanine dye represented by the formula (Ib) is preferably used in the form of a salt, which consists of the dye and an anion. The salt is described about the formula (I).
In the formula (I), R1 preferably is xe2x80x94NR21R24, xe2x80x94OR21 or xe2x80x94SR21. Where R1 is hydrogen, an aliphatic group or an aromatic group, the cyanine dye is quickly decolored by a base at an elevated temperature. However, the dye having hydrogen, an aliphatic group or an aromatic group as R1 is sometimes decolored while storing it because the dye is relatively labile. The stability of the dye is improved where R1 is xe2x80x94NR21R24, xe2x80x94OR21 or xe2x80x94SR21. The stable cyanine dye is represented by the formula (II). 
In the formula (II), X21 is xe2x80x94NR24xe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94. Each of R21 and R24 independently is hydrogen, an aliphatic group or an aromatic group, or R21 and R24 are combined to form a nitrogen-containing heterocyclic ring. R21 preferably is an aliphatic group or an aromatic group, and more preferably is an alkyl group, a substituted alkyl group, an aralkyl group, a substituted aralkyl group, an aryl group or a substituted aryl group. R24 preferably is hydrogen or an aliphatic group, and more preferably is hydrogen, an alkyl group or a substituted alkyl group. The nitrogen-containing heterocyclic ring formed by combining R21 and R24 preferably is a five-membered ring or a six-membered ring. The nitrogen-containing heterocyclic ring may contain a hetero atom (e.g., oxygen, sulfur) in addition to nitrogen.
In the formula (II), R22 is hydrogen, an aliphatic group or an aromatic group. R22 preferably is hydrogen or an aliphatic group, more preferably is hydrogen or an alkyl group, further preferably is hydrogen or an alkyl group having 1 to 15 carbon atoms, and most preferably is hydrogen.
In the formula (II), R23 is an aliphatic group. R23 preferably is a substituted alkyl group. In view of synthesis of the compound, R23 preferably is a substituted alkyl group having the same meanings as xe2x80x94CHR22xe2x80x94COxe2x80x94R21.
In the formula (II), L21 is a methine chain consisting of an odd number of methines. The number of the methines preferably is 3, 5, 7 or 9, more preferably is 3, 5 or 7, further preferably is 5 or 7, and most preferably is 5.
The methine may have a substituent group. Examples of the substituent groups include a halogen atom, an aliphatic group, an aromatic group, xe2x80x94NR25R26, xe2x80x94OR25 and xe2x80x94SR25. Each of R25 and R26 independently is hydrogen, an aliphatic group or an aromatic group. The aliphatic group and the aromatic group are defined above. The substituent groups of the methine can be combined to form an unsaturated aliphatic ring or an unsaturated heterocyclic ring. The unsaturated aliphatic ring is preferred to the unsaturated heterocyclic ring. The formed ring preferably is a five-membered or six-membered ring. Cycloheptene ring and cyclohexene ring are particularly preferred. The methine chain preferably is not substituted, or forms cycloheptene ring or cyclohexene ring by combining substituent groups.
In the formula (II), each of Z21 and Z22 independently is an atomic group forming a five-membered or six-membered nitrogen-containing heterocyclic ring. Examples of the nitrogen-containing heterocyclic rings include oxazole ring, thiazole ring, selenazole ring, pyrroline ring, imidazole ring and pyridine ring. A six-membered ring is preferred to a five-membered ring. The nitrogen-containing heterocyclic ring may be condensed with an aromatic ring. The nitrogen-containing heterocyclic ring and the condensed ring may have a substituent group. Examples of the substituent groups include a halogen atom, hydroxyl, nitro, carboxyl, sulfo and an alkyl group. Carboxyl and sulfo can be in the form of a salt. The cation forming a salt with carboxyl or sulfo preferably is an alkali metal ion.
The cyanine dye represented by the formula (II) is preferably used in the form of a salt, which consists of the dye and an anion. The salt is described about the formula (I).
In the formula (Ia), R11 preferably is xe2x80x94NR31R34, xe2x80x94OR31 or xe2x80x94SR31. The preferred cyanine dye is represented by the formula (IIa). 
In the formula (IIa), X31 is xe2x80x94NR34xe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94. Each of R31 and R34 independently is hydrogen, an aliphatic group or an aromatic group, or R31 and R34 are combined to form a nitrogen-containing heterocyclic ring. R31 preferably is an aliphatic group or an aromatic group, and more preferably is an alkyl group, a substituted alkyl group, an aralkyl group, a substituted aralkyl group, an aryl group or a substituted aryl group. R34 preferably is hydrogen or an aliphatic group, and more preferably is hydrogen, an alkyl group or a substituted alkyl group. The nitrogen-containing heterocyclic ring formed by combining R31 and R34 preferably is a five-membered ring or a six-membered ring. The nitrogen-containing heterocyclic ring may contain a hetero atom in addition to nitrogen.
In the formula (IIa), R32 is hydrogen, an aliphatic group or an aromatic group. R32 preferably is hydrogen or an aliphatic group, more preferably is hydrogen or an alkyl group, further preferably is hydrogen or an alkyl group having 1 to 15 carbon atoms, and most preferably is hydrogen.
In the formula (IIa), R33 is an aliphatic group. R33 preferably is a substituted alkyl group. In view of synthesis of the compound, R33 preferably is a substituted alkyl group having the same meanings as xe2x80x94CHR32xe2x80x94COxe2x80x94R31.
In the formula (IIa), L31 is a methine chain consisting of an odd number of methines. The number of the methines preferably is 3, 5, 7 or 9, more preferably is 3, 5 or 7, further preferably is 5 or 7, and most preferably is 5.
The methine may have a substituent group. Examples of the substituent groups include a halogen atom, an aliphatic group, an aromatic group, xe2x80x94NR35R36, xe2x80x94OR35 and xe2x80x94SR35. Each of R35 and R36 independently is hydrogen, an aliphatic group or an aromatic group. The aliphatic group and the aromatic group are defined above. The substituent groups of the methine can be combined to form an unsaturated aliphatic ring or an unsaturated heterocyclic ring. The unsaturated aliphatic ring is preferred to the unsaturated heterocyclic ring. The formed ring preferably is a five-membered or six-membered ring. Cycloheptene ring and cyclohexene ring are particularly preferred. The methine chain preferably is not substituted, or forms cycloheptene ring or cyclohexene ring by combining substituent groups.
In the formula (IIa), Y31 and Y32 independently is xe2x80x94CR37R38xe2x80x94, xe2x80x94NR37xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94Sexe2x80x94. Each of R37 and R38 independently is hydrogen or an aliphatic group or R37 and R38 are combined to form an aliphatic ring. The aliphatic group preferably is an alkyl group or a substituted alkyl group. The aliphatic ring preferably is a saturated aliphatic ring, more preferably is cyclopentane ring, cyclohexane ring or cycloheptane ring, and most preferably is cyclohexane ring.
In the formula (IIa), the benzene rings of Z31 and Z32 may be condensed with another benzene ring. The benzene rings of Z11, Z12 and the condensed ring may have a substituent group. Examples of the substituent groups include a halogen atom, hydroxyl, nitro, carboxyl, sulfo and an alkyl group. Carboxyl and sulfo can be in the form of a salt. The cation forming a salt with carboxyl or sulfo preferably is an alkali metal ion.
The cyanine dye represented by the formula (IIa) is preferably used in the form of a salt, which consists of the dye and an anion. The salt is described about the formula (I).
In the formula (IIb), R41 preferably is xe2x80x94NR51R52, xe2x80x94OR51 or xe2x80x94SR51. The most preferred cyanine dye is represented by the formula (IIb). 
In the formula (IIb), the two groups of X51 are identical. The two groups of R51 are also identical. X51 is xe2x80x94NR52xe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94. Each of R51 and R52 independently is hydrogen, an aliphatic group or an aromatic group, or R51 and R52 are combined to form a nitrogen-containing heterocyclic ring. R51 preferably is an aliphatic group or an aromatic group, and more preferably is an alkyl group, a substituted alkyl group, an aralkyl group, a substituted aralkyl group, an aryl group or a substituted aryl group. R52 preferably is hydrogen or an aliphatic group, and more preferably is hydrogen, an alkyl group or a substituted alkyl group. The nitrogen-containing heterocyclic ring formed by combining R51 and R52 preferably is a five-membered ring or a six-membered ring. The nitrogen-containing heterocyclic ring may contain a hetero atom in addition to nitrogen.
The cyanine dye represented by the formula (IIb) is preferably used in the form of a salt, which consists of the dye and an anion. The salt is described about the formula (I).
Examples of the cyanine dyes represented by the formula (Ib) are shown below. The anion (X) and R41 of the formula (Ib) are shown in the examples. 
The other cyanine dyes represented by the formula (I) are shown below. 